Web analytics company Compete reports that Google's social network saw a 43 percent jump in unique desktop visitors during the month of June, the highest growth it's had since September of 2011, and a total projected number of 31.9 million US visitors. The numbers are based on counting a sample of unique desktop-using visitors (mobile phones aren't included), then extrapolating based on the US population. That means these aren't official unique visitor numbers, but the jump in use is still pretty significant. It's not completely clear what precipitated it, but June was the month of Google I / O, as well as the month Google announced redesigned mobile apps that could have sparked interest in the service.
According to the same methodology, Facebook still dwarfs its competitor, but it didn't see a similar rise in usage. Hitwise, meanwhile, also saw a jump in Google+ engagement, recording the number of visits rather than unique visitors. These metrics aren't perfect, but they do shine some light on a service that's been difficult to get hard numbers on. One of the most well-known studies focused only on public posts, and Google's own engagement numbers could clock visit time for people who log into Plus only to switch to another service.